Cellular base stations typically include, among other things, a radio, a baseband unit, and one or more antennas. The radio receives digital information and control signals from the baseband unit and modulates this information into a radio frequency (“RF”) signal that is then transmitted through the antennas. The radio also receives RF signals from the antenna and demodulates these signals and supplies them to the baseband unit. The baseband unit processes demodulated signals received from the radio into a format suitable for transmission over a backhaul communications system. The baseband unit also processes signals received from the backhaul communications system and supplies the processed signals to the radio. A power supply may also be provided that generates suitable direct current (“DC”) power signals for powering the baseband unit and the radio. For example, the radio is often powered by a (nominal) 48 Volt DC power supply in cellular systems that are currently in use today. A battery backup is also typically provided to maintain service for some period of time during power outages.
In order to increase coverage and signal quality, the antennas in many cellular base stations are located at the top of a tower, which may be, for example, about fifty to two hundred feet tall. Until fairly recently, the power supply, baseband unit and radio were all located in an equipment enclosure at the bottom of the tower to provide easy access for maintenance, repair and/or later upgrades to the equipment. Coaxial cable(s) were routed from the equipment enclosure to the top of the tower that carried signal transmissions between the radio and the antennas.
FIG. 1 schematically illustrates a conventional cellular base station 10. As shown in FIG. 1, the cellular base station 10 includes an equipment enclosure 20 and a tower 30. The equipment enclosure 20 is typically located at the base of the tower 30, as shown in FIG. 1. A baseband unit 22, a radio 24 and a power supply 26 are located within the equipment enclosure 20. The baseband unit 22 may be in communication with a backhaul communications system 44. A plurality of antennas 32 (e.g., three sectorized antennas 32-1, 32-2, 32-3) are located at the top of the tower 30. Three coaxial cables 34 (which are bundled together in FIG. 1 to appear as a single cable) connect the radio 24 to the antennas 32. The antennas 32 are passive (unpowered) devices and hence none of the equipment at the top of the tower 30 require electrical power. Note that herein when multiple units of an element are provided, each individual unit may be referred to individually by the reference numeral for the element followed by a dash and the number for the individual unit (e.g., antenna 32-2), while multiple units of the element may be referred to collectively by their base reference numeral (e.g., the antennas 32).
In recent years, a shift has occurred and the radio 24 is now more typically located at the top of the tower 30 in new or upgraded cellular installations. Radios that are located at the top of the tower 30 are typically referred to as remote radio heads (“RRH”) 24′. Using RRHs 24′ may significantly improve the quality of the cellular data signals that are transmitted and received by the cellular base station as the use of RRHs 24′ may reduce signal transmission losses and noise. In particular, as the coaxial cable runs up the tower may be 100-200 feet or more, the signal loss that occurs in transmitting signals at cellular frequencies (e.g., 1.8 GHz, 3.0 GHz, etc.) over the coaxial cable may be significant. Because of this loss in signal power, the signal-to-noise ratio of the RF signals may be degraded in systems that locate the radio 24 at the bottom of the tower 30 as compared to cellular base stations where RRHs 24′ are located at the top of the tower 30 next to the antennas 32 (note that signal losses in the cabling connection between the baseband unit 22 at the bottom of the tower 30 and the RRH 24′ at the top of the tower 30 may be much smaller, as these signals are transmitted at baseband frequencies as opposed to RF frequencies).
FIG. 2 is a schematic diagram that illustrates a cellular base station 10′ according to this newer architecture. As shown in FIG. 2, the baseband unit 22 and the power supply 26 may still be located at the bottom of the tower 30 in the equipment enclosure 20. The radio 24 in the form of an RRH 24′ is located at the top of the tower 30 immediately adjacent to the antennas 32. While the use of tower-mounted RRHs 24′ may improve signal quality, it also, unfortunately, requires that DC power be delivered to the top of the tower 30 to power the RRH 24′. In some cases, the DC power may be delivered over a coaxial cable (not shown) that also carries communications between the baseband unit 22 and the RRH 24′. As shown in FIG. 2, more typically a fiber optic cable 38 connects the baseband unit 22 to the RRH 24′ (as fiber optic links may provide greater bandwidth and lower loss transmissions), and a separate power cable 36 is provided for delivering the DC power signal to the RRH 24′. The separate power cable 36 is typically bundled with the fiber optic cable 38 so that they may be routed up the tower 30 together.